Harsh environmental conditions promote cooperative behavior in an epiphytic fern

Harsh, unpredictable environments are known to favor cooperative groups in animals. Whether plants exhibit similar relationships is unknown. Staghorn ferns (<i>Platycerium bifurcatum</i>, Polypodiaceae) are epiphytes that form cooperative groups which build communal water and nutrient ‘nests’ at the tops of trees, a habitat characterized by water and nutrient stress. We conducted field observations to test whether staghorn ferns continue to live in large, reproductively active groups after they become dislodged from the canopy and fall to the forest floor, where they are less limited by water and nutrient deprivation. To rule out the potentially confounding effects of light limitation on the forest floor, we also conducted a multi-year glasshouse experiment where we transplanted individual plants into soil and onto vertically oriented boards under standardized light conditions. Results from field observations showed that dislodged colonies formed smaller groups that reproduced less than epiphytic colonies. Results from the glasshouse experiment showed that even when growing in full sun, terrestrial individuals tended to remain solitary, while epiphytic individuals tended to recruit new individuals into colonies. Results also showed that plants growing in potting soil and exposed to full sunlight sporulated more heavily than plants growing epiphytically. However, localities that are characterized by both elevated soil and light resources are generally not available to staghorn ferns in the wild, perhaps with the exception of large, epiphytic colonies with well-developed nests at the top of tree canopies. Overall results indicate that the harsh environmental conditions at the tops of trees trigger the formation of colonies in staghorn ferns, similarly to group living animals.

严苛且难以预测的环境已知会促使动物形成合作群体，而植物是否存在类似的协作关系尚不明确。鹿角蕨（*Platycerium bifurcatum*，水龙骨科Polypodiaceae）为附生植物，会形成合作群体并在树顶构筑兼具储水与储养功能的“巢穴”状结构，该生境的典型特征为水分与养分匮乏。本研究开展野外观测，以探究鹿角蕨从树冠脱落并坠落至林地后，是否仍能维持大型且具繁殖活性的群体——林地生境的水分与养分限制程度远低于树顶。为排除林地光照不足可能带来的混淆效应，本研究同时开展了多年期温室实验：在标准化光照条件下，将单株植株分别移栽至土壤中与垂直放置的木板上。野外观测结果显示，脱落的鹿角蕨集群形成的群体规模更小，繁殖能力也弱于附生状态的集群。温室实验结果表明：即便处于全光照环境中，陆生个体仍倾向于单独生长，而附生个体则会吸引新植株加入集群。实验还发现，栽培于盆栽土壤中且接受全光照的植株，其孢子产生量远高于附生状态的植株。但在自然环境中，兼具充足土壤养分与光照资源的生境通常并非鹿角蕨的可利用生境，仅树顶处发育完善的大型附生集群或许是例外。综合所有结果可知，树顶的严苛环境会诱导鹿角蕨形成集群，这与群居动物的生存策略高度相似。